Are they misty white/grey colour? if so, make sure you are pointing directly into them, and step a couple of feet backwards.Lumos them continously (It took me a while in the whomping willow mission at the start) and soon they will dissapear. MAKE SURE you have them in the same screen, or they will probably bite you. Hope this helps.

Charge your lumos spell and you'll see a small area iluminating the ground in front of you. Just move to make this lighty area in front of you reach the Gytrashes (the name of these dogs. Try looking for it in youtube.